An interest in energy storage devices has recently been growing.
Lithium ion rechargeable batteries, nickel-cadmium batteries, and nickel-metal hydride rechargeable batteries are widely used as power sources typically for portable intelligent communicators such as cellular phones and notebook-size personal computers, and for video cameras and portable music players. Among the batteries, lithium ion rechargeable batteries, which are superior in properties such as high energy density and high output density, have been rapidly investigated and developed since the debut thereof, and have established themselves as standard batteries for these consumer appliances.
With increase in functionality of these portable intelligent communicators, lithium ion rechargeable batteries (hereinafter also simply referred to as “batteries” (or “a battery”)) serving as power sources require further higher energy densities, i.e., require further higher capacities. In addition, they require longer cycle lives in consideration of environmental standpoints.
In general, a lithium ion rechargeable battery includes a cathode, an anode, a separator, and a nonaqueous electrolyte solution. For example, the cathode and the anode may each be prepared by mixing an active material, a conductive material for imparting electrical conductivity, and a binder for binding these components in a solvent, and applying the mixture to a current collector. The prepared cathode and anode are laid on each other via a separator, wound as a roll, inserted into a battery can (battery casing), in which an electrolyte solution containing an electrolytic salt dissolved in a nonaqueous solvent (organic solvent) is poured. Lids are then attached to the battery casing via insulating gaskets, and the battery casing is hermetically sealed to produce a battery.
Batteries thus prepared are often used at operating voltages of from 4.2 V to 2.5 V. Nickel-metal hydride rechargeable batteries, lead-acid batteries, and other batteries using water as the electrolyte generally have a limited rated voltage in the range of from 1.2 V to 2.0 V as a single cell, because water has a theoretical electrolytic potential of 1.229 V. The fact that a single cell of the lithium ion rechargeable battery can have a voltage higher than the theoretical electrolytic potential of water significantly owes to excellent electrochemical properties typically of the nonaqueous electrolyte solution and separator.
Such lithium ion rechargeable batteries may have higher capacities typically by increasing the mass of coating of the active material in the cathode and anode per unit area or by increasing the charge voltage. In addition to these measures, various techniques have been investigated to allow batteries to have higher capacities.
However, when repeatedly charged and discharged at a high capacity and at an upper limit of the operating voltage, a battery may suffer from capacity degradation, thus resulting in a shorter battery life. Independently, when stored and/or used in a high-temperature environment, gas is generated inside the battery to increase the inside pressure of the battery. This causes the battery to deform or causes the inner electrolyte to leak. The pressure rise also causes problems such as a malfunction in a safety mechanism which operates by the action of the pressure inside the battery.
To solve or avoid these problems, for example, Japanese Unexamined Patent Application Publication (JP-A) No. H08-236114 discloses a method for improving charge/discharge cycle properties of a battery by providing a metal oxide layer on the surface of a cathode active material. JP-A No. 2007-173064 discloses a technique for suppressing the decomposition of a nonaqueous electrolyte solution by providing a compound layer.
However, when a metal oxide layer is provided on the surface of an active material as in the technique disclosed in JP-A No. H08-236114, the presence of this layer impedes the diffusion of lithium ions and substantially prevents a current to pass through, resulting in a lower battery capacity.
Even when a layer capable of permeating lithium ions is formed as the technique disclosed in JP-A No. 2007-173064, the layer does not have an activity of suppressing gas generation inside the battery.
Accordingly, an object of the present invention is to provide a cathode active material, a cathode for a lithium ion rechargeable battery using the cathode active material, and a lithium ion rechargeable battery using the cathode, each of which less suffers from gas generation and thereby less causes battery swelling.